[R-390] AGC voltage issue

[Charles Steinmetz]

Larry wrote:

>I think the reason they are not rated higher is that the power 
>supply is a 'choke input' and the voltage regulator is active 
>immediately (putting enough load on the rectifier to minimize the 
>initial 'power on' high voltage condition you see in many other 
>power supplies).  Granted my 2 r390a's here have 26z5's

The regulator circuit does not draw anywhere near enough current to 
hold the initial voltage surge down (see previous posts in the 
archives about choke-input filters).  If your B+ does not exceed 
260v, it is because the 26Z5 cathodes warm up more slowly than the 
other tubes in the radio (the current drawn by all of the other 
tubes, even when they are only partially warmed up, is enough to 
provide a sufficient load to hold the B+ down with the choke-input 
filter).  And that is how it is supposed to work.  However, some 
26Z5s warm up much quicker, and if they have been replaced with SS 
diodes the rectifiers are active immediately, so the voltage rises 
until all of the tubes have warmed up and are conducting.  (Also note 
that if you switch the radio to Standby, the B+ rises because the 
load is removed.)

Depending on how you measure the B+, it is also possible that the 
indication you see may lag and integrate the actual voltage (i.e., 
your measurements could be under-reporting the actual peak).  The 
time constant of a VTVM can miss this transient (due to both the R-C 
noise filtering and the electromechanical ballistics of the 
d'Arsonval movement).

Best regards,

Charles


ps.  I don't understand why so many people seem to want to get by 
with substandard parts, and why there is so much resistance to doing 
things right (e.g., using a capacitor that is rated for less voltage 
than it may see in operation, or using metallized film rather than 
film-and-foil caps where the voltage could exceed 200v).  Jeez, 
people, just buy the right part and use it.  IMLTTO, anyone who 
whines about the cost of the right capacitor, or that they can't fit 
it into the old can, doesn't deserve to own one of these fine 
radios.  They should sell it to someone who will treat it right and 
take up stamp collecting.